The background of the invention will be discussed in two parts: 1. Field of the Invention
This invention relates to moisture monitoring devices, and more particularly to devices for monitoring moisture in a continuous flow high pressure natural gas line.
2. Description of the Prior Art
In high pressure natural gas transmission systems, the natural gas is passed through dehydration apparatus, such as glycol amine moisture absorption treatment units, before delivery to the pipeline. With line pressures in the range of 600 to 800 pounds per square inch, moisture must be reduced to a level below 8 lbs/million cubic feet to avoid condensation at cold locations on the pipeline, or formation of hydrocarton hydrates at such locations.
Dehydration units may operate at high ambient temperature at the production location which can result in low water vapor absorption efficiency during some seasons. Condensation at cold locations on the pipeline can thus occur. It is, therefore, necessary to continuously measure the concentration of water in the natural gas.
Prior art methods of measuring moisture or water content of gases have utilized electrolysis cells and operate on the principle that water can be electrolized by passing direct current through a film of moist phosphorus pentoxide between platinum electrodes. In such instruments a regulated amount of sample gas is flowed through the cell. As water is decomposed to hydrogen and oxygen by electrolysis, a current proportional to the concentration of water vapor in the natural gas at the rate of flow of the natural gas is recorded. In such systems, however, accuracy is highly dependent on the degree of accuracy employed in regulation of the rate of flow of the gas. Furthermore, during periods of excess moisture, cell overload problems are encountered.
As an alternative, solid state moisture sensing elements have been developed for incorporation in electronic circuits, with such elements changing capacitance of the circuit in response to changes in water vapor pressure. However, with such solid state elements, inaccuracies arise as a consequence of contaminants in pipeline gas. Furthermore, with such elements, periodic calibration is required to provide proper measurement of moisture content.
As a means of avoiding regulation of sample flow rates, a number of instrument systems have been developed employing deliquescent salt compounds, such as lithium chloride and lithium bromide. Such salts absorb water vapor at very low pressure to form a conductive solution. The vapor pressure of water in equilibrium with the salt is inversely proportional to the temperature of the sensor in the sample gas chamber. Some instruments of this type utilize conductive electrodes in electrical contact with glass fabric impregnated with one of the salts, with the electrodes connected in an alternating current circuit in series with a resistance heater, supported under, and insulated from, the impregnated fabric. In this system, moisture is absorbed in the salt, forming a conductive film between the electrodes, which, in turn, reduces the resistance in series with the heating element, and correspondingly increases the current through the heating element to increase the heat to therby increase the salt film temperature. As the salt temperature increases, the resistance of the salt film increases due to the increased evaporation of moisture until the resistance of the salt film limits current to the heating element at a level required to sustain the operating temperature. A thermal element records the elevated dew point thus produced. However, in such systems, as a result of the high operating currents normally occurring during high moisture periods, sensor failure is not uncommon with erosion of electrodes and sputtering of hot solutions being encountered.
Exemplary of the prior art is U.S. Pat. No. 2,629,253, issued to Deaton on Feb. 24, 1953 for a "Moisture Content Recorder for Gases Under pressure". In accordance with the teachings of this patent, a sample gas chamber is provided for receciving a preferably purified gas sample, with the chamber being formed in a metallic block having means for creating isothermal conditions within the chamber. A cylindrical member is contained within the chamber and provided with a pair of concentric coils, preferably of palladium wire, wound thereabout in noncontacting relation with each other. The cylindrical element is coated with a deposit of lithium chloride or other suitable salt whose electrical resistivity varies with the moisture content of the surrounding atmosphere. The coils are then electrically connected in a bridge circuit for providing an indication of moisture content at the predetermined isothermal condition.
Although not directed to the extreme environment associated with pipeline moisture content measurements, a prior art system for continuously monitoring ambient relative humidity is shown and described in U.S. Pat. No. 2,733,607, issued Feb. 7, 1956 to Miller for a "Relative Humidity Measuring Instrument". In accordance with the system of this patent, relative humidity sensitive resistance elements are combined with a thermometric, temperature sensitive resistor in a bridge circuit, with unbalances in the bridge being used to drive a motor which in turn positions rheostat wipers to rebalance the bridge, at which point motor operation ceases. The relative humidity sensing element includes a grid of two combs of gold leaf on a substrate, with the teeth of the combs interleaved but not contacting, with a coating of lithium chloride or other suitable hygroscopic material on the substrate.
U.S. Pat. No. 3,516,282, issued June 23, 1970 to Leach et al for a "Dewpoint Detection Apparatus", the system including a fiberglass tube impregnated or coated with a lithium chloride with parallel coils of gold wire wound thereabout. A power supply circuit operating from AC line voltage is used to drive an oscillator, with the coils being connected to the oscillator output having a frequency between 1 kHz and 10 kHz. The alternating current from the oscillator is supplied to the coils from the secondary of a transformer having a first capacitor in series therewith and a second capacitor in parallel therewith to provide parallel and series resonance at various portions of the curing cycle of the system to essentially provide constant power to the coils during the time when excess moisture is being expelled. At equilibrium conditions, the power supply reverts to constant voltage. A second transformer is in series with the output of the first transformer to control an amplifier, which in turn, controls a heater element. A temperature sensitive resistor is contained within the tube for providing an indication of temperature. The use of line voltage and transformers tends to introduce variables into the system, thus reducing accuracy in output measurement.
Another "Humidity Sensor" is shown and described in U.S. Pat. No. 4,041,437, issued Aug. 9, 1977 to Matsuura et al, the sensor being a solid state device with a negative coefficient of resistivity for relative humidity, comprising 99.99 to 10 mole percent of iron oxide and 0.01 to 90 mole percent of at least one member selected from alkali metal oxides, which are lithium oxide, sodium oxide, potassium oxide, potassium oxide and cesium oxide.
An ambient environment humidity measuring system is shown and described in U.S. Pat. No. 4,419,988, issued Dec. 13, 1983 to Kitamura et al, such patent being entitled "Humidity Measuring Method". In this method a current is applied to a thermistor to heat the same to a temperature above open air temperature with the thermistor held in the open air, with the change in resistance value being detected from which the humidity of the air is determined.
A three-terminal semiconductor sensor device for relative humidity measurement circuits is shown and described in U.S. Pat. No. 4,379,406, issued to Bennewitz et al on Apr. 12, 1983, the patent being entitled "Relative Humidity Detector Systems and Method of Increasing the Calibration Period of relative Humidity Detector Systems". The sensor utilizes an essentially pure aluminum oxide layer which has a disordered crystalline structure and a density gradient which varies from a low density at its lower surface to a higher density at its upper surface which together produce the desired linear response to relative humidity.
It is accordingly an object of the present invention to provide a new and improved moisture monitoring apparatus for use in high pressure gas pipeline.
It is another object of the present invention to provide a new and improved moisture monitoring apparatus for use in a high pressure gas pipeline for providing an indication of moisture independent of flow rate of the gas.
It is still another object of the present invention to provide a new and improved moisture monitoring apparatus having solid state control and measurement circuitry with direct current control of the heater thereof.
It is a further object of the present invention to provide a new and improved moisture monitoring apparatus which is highly reliable and accurate.